1. Field of the Invention
The present invention generally relates to image forming apparatuses and more particularly relates to supply of toner to a developing unit.
2. Description of the Related Art
When two-component developer that circulates in a developer circulation path in a developing device is used in image formation, toner in the two-component developer is consumed in the image formation. The amount of toner equivalent to the consumed toner is supplied (added) to the two-component developer by the toner supplying unit. The following two methods are typically used to supply toner to the two-component developer.
In the first method, the amount of toner that would be consumed in the process of development of latent images is calculated (predicted) using pixel-writing information (image information) that is used when an exposing unit (latent image forming unit) forms the latent images on a latent image carrier. Then, the amount of toner equivalent to the calculated consumption amount of toner is supplied to the two-component developer in one shot, or supplied in small portions intermittently at regular intervals.
In the second method, a toner-density sensor (toner density detecting unit) is arranged at a predetermined location (predetermined detection location) of a screw conveyor (developer conveying unit), which is used for circulating two-component developer in the developing device, and toner density at the detection point is measured by the toner-density sensor. Then, toner is supplied to the two-component developer in one shot or supplied in small portions intermittently at regular intervals in such a manner that the toner density reaches a predetermined target toner density.
In both methods, however, toner is supplied to the two-component developer in one shot or supplied in small portions intermittently at regular intervals. This makes it difficult to solve the problem of uneven toner density of the two-component developer that circulates in the circulation direction in the developing device (hereinafter, simply referred to as “uneven toner density”). Detailed description is given below with reference to drawings.
FIG. 27 is a schematic diagram of an exemplary configuration of the developing device. In the developing device, two screw conveyors, a first screw conveyor 8 and a second screw conveyor 11, circulate two-component developer in the direction indicated by the arrow A. In the developer circulation path, a developing roller 12 is arranged opposite a portion in which the second screw conveyor 11 is arranged, where the two-component developer is raised onto a surface of the developing roller 12 and the two-component developer that passes through a development region returns thereto. In the developer circulation path, a toner supply port 17 is arranged above a portion in which the first screw conveyor 8 is arranged, and toner is supplied to the developing device from the toner supply port 17 by a toner supplying unit (not shown). Uneven toner density is measured by a toner-density sensor at a point indicated by reference symbol B in FIG. 27.
FIG. 28 is a graph of the relation between toner supply and uneven toner density when toner is supplied to the two-component developer in one shot. FIG. 29 is a graph of the relation between toner supply and uneven toner density when the toner is intermittently supplied to the two-component developer at regular intervals.
In FIGS. 28 and 29, each of the waveforms (consumption waveforms) indicated by the thin line represents a measurement result of toner density, which is measured by the toner-density sensor without supplying additional toner, of the two-component developer after the two-component developer with uniform toner density has been used for developing a predetermined latent image. In other words, the consumption waveform represents an example of the uneven toner density that occurs after the development.
Each of the waveforms (supply waveforms) indicated by the dotted line represents a measurement result of toner density, which is measured by the toner-density sensor, of the two-component developer after the two-component developer with uniform toner density has been used for developing the predetermined latent image and after toner is supplied to the two-component developer using each of the methods.
Each of the waveforms indicated by a two-dot chain line in FIG. 29 represents each of the supply waveforms of toner supplied intermittently, and the supply waveform indicated by the dotted line represents a combination of each of the supply waveforms indicated by the two-dot chain line.
Each of the waveforms indicated by the heavy solid line is a combination of the consumption waveform and the supply waveform and indicates uneven toner density of the two-component developer after it has been used for developing a predetermined latent image and after toner is supplied to the two-component developer using each of the supply methods.
As shown with heavy solid lines in FIGS. 28 and 29, in the method in which the toner is supplied to the two-component developer in one shot (the first method) and in the method in which the toner is intermittently supplied to the two-component developer at regular intervals (the second method), uneven toner density is still present in the two-component developer after the toner is supplied thereto.
At the time of actual image formation, a consumption waveform is not uniformly produced because the consumption waveform varies according to the formed image, i.e., the position or the size of the developed latent image. Accordingly, as in the conventional method, when the toner is supplied at regular intervals and at a constant rate regardless of the variation of the consumption waveform, the problem of uneven toner density of the two-component developer that occurs after the toner is supplied cannot be eliminated.
This point is explained in more detail below. In the developing device shown in FIG. 27, the two-component developer that is conveyed by the second screw conveyor 11 is conveyed along the developer circulation path in a direction orthogonal to the direction in which the two-component developer is conveyed by the developing roller 12, adheres to a surface of the developing roller 12, and is conveyed to a development region. After being used for development at the development region, the two-component developer returns to the developer circulation path and is conveyed by the second screw conveyor 11.
When the latent images are unevenly distributed on the latent image carrier, the two-component developer that has been used for development possibly in a state where it contains a portion that consumes a large amount of toner and a portion that scarcely consumes toner. The two-component developer having that state returns to the developer circulation path. In such a case, uneven toner density occurs in the two-component developer after the two-component developer returns to the developer circulation path. Furthermore, the state of the uneven toner density varies according to the distribution state of the latent image on the latent image carrier.
FIG. 30 is a graph of the relation between distributions of the latent images on the latent image carrier and states of uneven toner density. In FIG. 30, the arrow A indicates the conveying direction in which the two-component developer is conveyed by the second screw conveyor 11. The arrow C indicates the moving direction of the surface of the latent image carrier.
The upper part of FIG. 30 represents three image patterns that are formed on three recording media. The lower part of FIG. 30 is a graph that represents measurement results (consumption waveform) of the toner density, which are measured by the toner-density sensor without supplying additional toner, of the two-component developer after the two-component developer with uniform toner density has been used for developing latent images corresponding to each of the image patterns.
As shown in FIG. 30, the consumption waveforms, i.e., states of uneven toner density, vary according to the distributions of the latent images on the latent image carrier. In FIG. 30, when comparing the image pattern shown on the left side (hereinafter, “left pattern”) with the image pattern shown on the right side (hereinafter, “right pattern”), the consumption waveform of the right pattern is broader than that of the left pattern. This is because the two-component developer that develops the right pattern is conveyed a longer distance from a point where the two-component developer that consumes the toner returns to the developer circulation path to the measurement point B for the toner-density sensor and stirred for a longer period by the screw conveyor. In other words, the two-component developer that develops the right pattern is stirred for a longer period than the two-component developer that develops the left pattern during the period of time in which the two-component developer that consumes the toner is conveyed to the measurement point B for the toner-density sensor. This slightly cancels out toner density compared with the two-component developer that develops the left pattern, resulting in broader consumption waveform.
FIG. 31 is a graph that further explains the relation between positions of the latent images on the latent image carrier and states of uneven toner density.
FIG. 31 illustrates three image patterns on which latent images are formed at three different positions in the conveying direction of the two-component developer that is conveyed by the second screw conveyor and two image patterns on which latent images are formed at two different positions in the moving direction of the surface of the latent image carrier. Parts of the image patterns are overlapped. The image patterns have the same image area.
The graph illustrated in the lower part of FIG. 31 represents measurement results (consumption waveform) of toner density, which are measured by the toner-density sensor without supplying additional toner, of the two-component developer after the two-component developer with uniform toner density has been used for developing the latent images that correspond to each of the three image patterns. The graph illustrated in the left part of FIG. 31 represents measurement results (consumption waveform) of toner density, measured by the toner-density sensor without supplying additional toner, of the two-component developer after the two-component developer with uniform toner density has been used for developing the latent images that correspond to each of the three image patterns.
When latent images having the same area are formed at different positions in the conveying direction of the two-component developer that is conveyed by the second screw conveyor, as shown in the lower part of FIG. 31, the consumption waveforms corresponding to the image patterns have a different peak timing, half width (broad state), and minimum toner density. As described above, this is caused by a difference in the conveying distance of the developer between a point where the two-component developer that consumes the toner returns to the developer circulation path and the measurement point B for the toner-density sensor. Specifically, the difference in peak timing is simply caused by the difference in time it takes the two-component developer that consumes the toner to reach the measurement point B. The differences in the half width (broad state) and the minimum toner density are caused by a difference in the amount of two-component developer that consumes the toner stirred during which two-component developer reaches the measurement point B for the toner-density sensor.
When latent images are formed at different positions in the moving direction of the surface of the latent image carrier, as shown in the left part of FIG. 31, the consumption waveforms corresponding to each of the image patterns have different peak timing but have the same half width (broad state) and the same minimum toner density. This is because the two-component developer that consumes the toner used for the image patterns returns to the same position in the developer circulation path, whereby the conveying distances of the developer to the measurement point B are the same. Accordingly, the amount of two-component developer stirred is the same as that stirred during the period in which the two-component developer reaches the measurement point B. That is, the half width (broad state) and the minimum toner density are the same for the image patterns. However, the timing by which the two-component developer that consumes the toner returns to the developer circulation path differs, whereby the peak timing differs accordingly.
As described above, the consumption waveform is not uniformly produced during actual image formation because the consumption waveform varies according to the size of the latent image formed on the latent image carrier and the position of the latent image. Accordingly, in the conventional method, although the average toner density of all of the two-component developer in the developing device can be maintained at a target toner density, it is difficult to reduce uneven toner density of the two-component developer.
Japanese Patent Application Laid-open No. H11-219015 discloses a method of supplying toner in order to reduce uneven toner density in a developing device that is configured to separately control a supply amount of toner that is supplied from a plurality of toner supply ports on the basis of a result of histogram analysis obtained from density distribution of image data. Using this method, uneven toner density of the two-component developer can be eliminated.
Japanese Patent Application Laid-open No. 2006-171177 discloses a method of supplying toner in order to reduce uneven toner density in a developing device with a configuration in which image data is split into a finite number of divisions, and toner is supplied from toner supplying units corresponding to the divisions based on the number of dots in the division.
In the method of supplying toner disclosed in Japanese Patent Application Laid-open No. H11-219015, to eliminate uneven toner density, the amount of toner supplied from the toner supply ports needs to be separately controlled. Specifically, in an embodiment disclosed in Japanese Patent Application Laid-open No. H11-219015, six toner supply ports are arranged, and the supply amount of toner is separately and simultaneously controlled for these six ports. It is actually impossible to eliminate uneven toner density without the configuration in which the supply control of toner is separately and simultaneously performed for that number of toner supply ports.
To separately perform supply control for a plurality of toner supply ports, driving sources that drives toner supplying members for supplying toner from each of the toner supply ports need to be separately arranged for each toner supply port. When compared with a case of using a typical apparatus in which only one driving source for supplying toner is arranged, there is a problem of an increase in the size of apparatuses because the positioning space for the a plurality of driving sources is required or there is an increase in costs for parts required by the plurality of driving sources.
In the method of supplying toner disclosed in Japanese Patent Application Laid-open No. 2006-171177, a plurality of toner supply ports also need to be separately controlled; therefore, the same problem in the technology disclosed in Japanese Patent Application Laid-open No. 2006-171177 occurs.